Devices for acquiring and transmitting at least one measurement signal that characterizes the state of a vehicle tire, such as tire air pressure and/or tire air temperature, are in particular tire pressure monitoring devices (Tire Pressure Monitoring System, TPMS), and are used to monitor tire air pressure in vehicles in order to prevent tire failure due to too-low tire air pressure, thus reducing the number of accidents caused by tire failures.
If a vehicle is operated with too-low tire air pressure, this causes increased flexing at the tire edges, thus increasing the wear of the tire. At high acceleration, a tire weakened in this way can under some circumstances no longer withstand the load, and may burst. In addition to tire damage, slow-acting leaks, due to the diffusion of gas through the tire rubber over time, are a significant cause of low tire pressure.
However, tire filling pressure is not only an important factor in driving safety. Driving comfort, tire lifespan, and fuel consumption are all significantly influenced by the filling pressure. A pressure reduced by 0.6 bar can increase fuel consumption by up to 4%, and can shorten the lifespan of the tire by up to 50%.
The increasing proportion of tires having emergency running capacity also requires the use of tire pressure monitoring systems, because the driver can no longer recognize a tire having significantly low pressure on the basis of driving response. In order to prevent the driver from unknowingly exceeding speed and distance limits that hold in such a case, the emergency-running tires must be used only in combination with tire pressure monitoring systems or pressure-loss warning systems.
A fundamental distinction is made between two types of tire pressure monitoring systems: direct and indirect systems.
In direct systems, a sensor module having a pressure sensor is installed in each tire of the vehicle. This sensor uses a coded radio-frequency transmission path to transmit data from inside the tire, such as air pressure and air temperature, to a control device. In the control device, these data can be evaluated, so that, in addition to pressure loss in individual tires, slow loss of pressure in all the tires (e.g. due to diffusion through the rubber) can also be recognized. If the tire air pressure sinks below a defined threshold, or if the pressure gradient exceeds a determined value, the driver is warned by an optical or acoustic signal. As a rule, the sensor modules are supplied with power by a battery. In comparison to other applications, this results in additional requirements with regard to power consumption, media resistance, and sensitivity to acceleration. Micromechanical absolute pressure sensors can be used as sensor elements.
The data measured in the tire by the pressure and temperature sensor are prepared in the sensor module, are modulated onto an RF carrier signal, and are radiated via an antenna. This signal is detected either via individual antennas at the wheel housings or in a central receiver (e.g. in the control device of existing remote keyless entry systems).
In indirect systems, a pressure loss in the tire is determined not immediately but rather via a derived quantity. For this purpose, a mathematical-statistical evaluation is carried out of the differences in rotational speeds among all the wheels. In vehicles having ABS systems, the wheel rotational speed required for this is determined using sensors that are already present, and is transmitted to the ABS control device. Differences in rotational speed occur when the diameter of the corresponding tire becomes smaller due to pressure loss, so that its rotational speed increases relative to the other three tires. Using difference formation, which can be realized using a low-cost expansion of the ABS software algorithms, larger pressure losses at up to three tires can be recognized.
Disadvantages of the indirect system are its comparatively long detection time periods (10 minutes) and a high detection threshold compared to direct systems. Indirect systems currently on the market are not capable of detecting slow pressure losses due to diffusion in all four tires. Another disadvantage of direct systems relative to indirect systems is their significantly higher cost and their limited lifespan, due to the use of a battery.
A vehicle of the general type indicated is discussed in German patent document DE 103 01 192 A1. Due to the fact that the energy supply device includes at least one generator that rotates together with the vehicle tire and at least one magnetic field generator at the vehicle, and that an electrical voltage is produced in the generator by electromagnetic induction, batteries can be completely done without for energy supply. In addition, such a system has a significantly lower maintenance requirement due to the fact that no battery has to be changed.
In comparison, the object of the present intention is to further develop an energy supply device for a device for acquiring and transmitting at least one measurement signal that characterizes the state of a vehicle tire, such as tire air pressure and/or tire air temperature, in such a way that on the one hand it can be easily installed on the vehicle and on the other hand it has a long lifespan.
According to the exemplary embodiments and/or exemplary methods of the present invention, this object is achieved by the features described herein.